A semiconductor device typically includes a semiconductor chip that is mounted on a die attach pad of a lead frame, the lead frame having a number of leads that extend inwards from its edges. During fabrication of the semiconductor device, electrical connections are established between bond pads on the semiconductor chip and predetermined ones of those leads. This is achieved through a wire bonding process in which one end of a wire is attached to each bond pad of the semiconductor chip and the other end of the wire to a point on the appropriate lead. There are several wire bonding techniques available in the industry, for instance ultrasonic bonding and thermosonic (combined ultrasonics and thermocompression) bonding, whereby the wire is welded to the appropriate bond pads and leads.
The lead frame is typically made from a stamped metal sheet and is usually produced in a strip or ribbon where leads of a particular configuration are repeated throughout the whole length of the lead frame strip. During the wire bonding process, strips of lead frames, in which the semiconductor chips are already mounted, are loaded into a magazine in a wire bonding apparatus. They are taken out of the magazine and positioned for wire bonding to take place in each lead frame. After the wire bonding process has been completed, the semiconductor devices are sent for further processing and packaging.
The wire used is usually of a small diameter and has to be electrically conductive. Generally, most wires used for bonding are typically made of gold, aluminum alloy or copper. A clamping device is typically employed to hold the lead frame, more particularly to hold the leads of the lead frame down against a heater plate as the wire bonding process takes place. This is to ensure correct placement and efficient heat transfer. The structure of such clamps varies, but they include a surface or bodies to hold down the leads.
For instance, the clamp member may have a rigid body with a hole through it and a ridge, extending below the main surface of the body, around the edge of the hole. The hole allows clearance for the semiconductor chip and access for the bonding machine. The ridge holds down the leads in position. In other designs, the ridges may be replaced with rigid downward protrusions.
The wire used in wire bonding is usually of a small diameter. The bond pads are small and the leads are small. Everything has to be positioned accurately.
Unfortunately, this conventional approach fails to take into account variations in thicknesses and widths and positions due to manufacturing tolerances, for instance variations in lead thickness, variations that accumulate when the various clamping parts are assembled and used (i.e. from the clamp post, clamp post holder, heater plate, clamp, lead frame, etc.). Also it takes no account of variations introduced by repeated use of the clamp. These are the result of wear and tear and also deformation resulting from repeated heating and cooling cycles. In order to be able to achieve optimal clamping of the leads to overcome fabrication tolerances, the clamp member has to be offset by manually filing the clamp member in various areas. This is extremely time consuming and may take from hours to days depending on the complexity of the problem. Even then, quite soon such a clamp member would be rendered almost ineffective and the quality of the wire bonds would suffer.
One prior variation of the above mentioned conventional approach of employing a clamping device to hold the lead frame is to use clamping inserts, with ridges around holes and pivotally mounted in a clamping frame on leaf springs. The frame is supported at both ends. This approach at least tries to overcome the problem of deformities in the frame, but does not allow for variations in the thickness of the leads.
Another prior approach uses a clamp frame, which is supported at both ends with interchangeable clamp inserts. One such insert includes a round hole into which a number of identical spring-like fingers are radially directed at equal spacings. These are also directed downwards (at an angle of between 30 and 45 degrees and always less than 90 degrees) below the level of the frame and rest of the insert. The apparent intent is for each lead to be clamped independently and uniformly.
Such clamp inserts, however, tend to be thin and are difficult to manufacture evenly. Moreover, they deform upon tightening onto the frame. Shims then have to be inserted between the insert and the holder to compensate for the differences. This is a tedious optimizing process that may take days. Further, with a thin structure such as this, movement of one finger affects the others and hence the fingers cannot move independently. Additionally, the structure adds to the thickness of the clamp.
Another problem with this approach is the uniformity of the fingers. The ideal point of clamping for leads around a chip is not always exactly the same distance away from the center of the chip. Indeed, the leads usually have a rectangular arrangement. Sometimes the clamping point will be too far away from the ideal point. Sometimes a finger will fall between two leads and clamp neither satisfactorily, possibly even leading to deformation of a lead.
In view of the foregoing, it is desired to produce a new clamp, preferably a new one that avoids or at least partially alleviates at least some of the above problems.